Powder coatable epoxy composition and post-tensioning cable coated therewith

ABSTRACT

Thermosetting powder coatings are provided which comprise a high epoxide equivalent weight epoxy resin or mixture of epoxy resins, a cure catalyst for the epoxy resin, an acrylic resin and a polyamide resin. These powder coatings are useful for forming corrosion resistant, essentially crack-free coatings on metal cable, especially cable which is subjected to post-tensioning.

This is a divisional of copending application Ser. Nos. 681,521 filed on12/14/84 and 015,241 filed on 2/17/87, now U.S. Pat. No. 4,761,336.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to compositions useful as thermosetting powdercoatings. These compositions are used as protective coatings for cableswhich require tensioning or stressing after the cable has been coatedand the coating cured. These "post-tensioned" cables are used, e.g., forstructural support in bridges.

2. Description of the Prior Art

Steel cable used for structural support in bridges is subjected totensioning in concrete. Since these cables are subject to corrosion theyare generally coated with a material which will provide corrosionresistance. Such coatings, however, have been susceptible to crackingduring tensioning of the cable since they lack the flexibility requiredto withstand the tensioning forces. Once the coating developed thesepost-tensioning cracks, its ability to protect the cable againstcorrosion was severely reduced. Thus, it would be highly desirable tocoat the cable with a material which does not crack upon tensioning ofthe cable.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided athermosetting powder coating composition comprising a high epoxideequivalent weight epoxy resin or mixture of epoxy resins, a curecatalyst for the epoxy resin, an acrylic resin and a polyamide resin. Ithas been discovered that the thermosetting powder coating compositionsof this invention provide coatings which have sufficient flexibility toprevent cracking of the coating and are, therefore, useful for providingessentially crack-free, corrosion resistant coatings on cable,especially post-tensioned cable (i.e., cable which is tensioned aftercoating). This invention further provides a process for protecting ametal cable against corrosion comprising applying to the cable athermosetting powder coating composition comprising a high epoxideequivalent weight epoxy resin or mixture of epoxy resins, a curecatalyst for the epoxy resin, an acrylic resin and a polyamide resin,and curing said powder coating said cable, whereby an essentiallycrack-free coating is formed on the cable. Following curing of thecoating the cable can be tensioned and the coating will remainessentially crack-free.

Also provided by this invention is a corrosion resistant, metal cablecomprising a metal cable coated with an essentially crack-free layer ofa cured thermosetting powder coating which in its uncured statecomprises a high epoxide equivalent weight epoxy resin or mixture ofepoxy resins, a cure catalyst for the epoxy resin, an acrylic resin anda polyamide resin. The cable may also be in the tensioned state with thecoated layer remaining essentially crack-free.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The high equivalent weight epoxy resins useful in the thermosettingpowder coatings of this invention are solid resins which are thereaction products of a diol and a halohydrin. Epoxy resins which areuseful in the practice of this invention are exemplified by, but are notlimited to, resins produced by reacting epichlorohydrin and bisphenol A.Preferred epoxy resins include those sold by Shell Chemical Companyunder the designation "EPON", such as EPON Resin 1007F, EPON Resin 1009Fand EPON Resin 1004, all of which are4,4'-isopropylidenediphenol-epichlorohydrin resins.

The epoxy resin component of the thermosetting powder coatingcompositions of this invention has a high epoxide equivalent weight.Typically, the epoxide equivalent weight of the epoxy resin componentwill be at least about 2250. When mixtures of epoxy resins are employedin the epoxy resin component, the epoxide equivalent weights of some ofthe individual epoxy resins may be below about 2250, but the epoxideequivalent weight of the mixture, i.e., the weighted average of theepoxide equivalent weights of the individual resins, should be at leastabout 2250. Thus, a typical epoxy resin component of the powder coatingsof this invention may comprise:

    ______________________________________                                                    EPOXIDE EQUIVALENT                                                                              PARTS BY                                        EPOXY RESIN WEIGHT (APPROX.)  WEIGHT                                          ______________________________________                                        EPON Resin 1007F                                                                          2000              50.0                                            EPON Resin 1009F                                                                          2800              50.0                                            EPON Resin 1004                                                                            910              10.0                                            ______________________________________                                    

which provides an epoxy resin component with a weighted average epoxideequivalent weight of about 2265.

It is believed that the high equivalent weight of the epoxy resincomponent is at least partially responsible for the excellentflexibility or elongation of the cured powder coating.

The epoxy resin reacts via homopolymerization during curing of thecoating. Generally, a catalyst is required to permit this reaction tooccur at a commercially acceptable rate. A particularly preferred curecatalyst is 2-methylimidazole. It is believed this catalyst promotes theformation of ether linkages during the homopolymerization of the epoxyresins via the following reaction mechanism: ##STR1## It is furtherbelieved that, along with the aforementioned high epoxide equivalentweight, these ether linkages contribute to the flexibility of the curedcoating.

The acrylic resins which may be employed in the thermosetting powdercoating compositions of this invention are generally liquids which havebeen converted to powder form by absorption onto silica-type materials.These acrylic resins function as melt flow control agents. Exemplary ofthe acrylic resins is Resiflow P acrylic resin sold by SBS Chemicals,Inc. which is a 2-propenoic acid, ethyl ester polymer.

In accordance with the present invention, the thermosetting powdercoating compositions also contain a polyamide resin adhesion promoter.Examples of such polyamide resins include, but are not limited to, thosesold by General Mills Chemicals, Inc. under the designation "Versamid"which are the condensation products of polyamines and the dibasic acidsobtained when certain unsaturated fatty acids are polymerized, e.g. the9, 12-octadecandienoic acid (Z,Z)-, dimer, polymer with1,2-ethanediamine. Versamid 754 polyamide is a preferred polyamideresin.

In addition to the the above components, the thermosetting powdercoatings of this invention may contain additional additives. Forexample, a coloring agent may be added to the powder coating compositionshould a colored coating be desired. Fumed silica may also be employedas a powder flow additive. A preferred fumed silica is sold by CabotCorporation under the trademark CAB-O-SIL. Other possible additivesinclude slip or antimar additives, such as dimethyl, methyl phenethyland trimethylsilyl-terminated siloxanes and silicones, or polyethylene.

The components of the thermosetting powder coating compositions of thisinvention are employed in the amounts indicated in Table A below.

                  TABLE A                                                         ______________________________________                                                    PARTS BY WEIGHT                                                   COMPONENTS    GENERAL    PREFERRED                                            ______________________________________                                        Epoxy resin   80     to 130  90     to 120                                    Cure catalyst 0.1    to 3.0  0.4    to 0.8                                    Acrylic resin 0.5    to 3.0  1.0    to 2.0                                    Polyamide resin                                                                             0.5    to 10.0 0.75   to 2.0                                    Coloring agent                                                                              0      to 50.0 0.1    to 1.5                                    Fumed silica  0      to 1.0  0.1    to 0.3                                    ______________________________________                                    

The thermosetting powder coating compositions of this invention areprepared by conventional techniques employed in the powder coating art.The components, with the exception of the powder flow additive, areblended together thoroughly. The resulting mixture is then melt mixed,for example in an extruder or on a two roll mill. The resulting product,usually in sheet form, is broken into chips, and blended with the powderflow additive in a low intensity blender. The resulting product is thenground to a powder on, for example, a hammermill and sieved to achievethe desired particle size.

The powder coating composition of this invention may be applied to thecable and cured using standard techniques employed in the powder coatingart. For example, the cable may be heated to about 450° F. and then thepowder coating can be applied electrostatically. The residual heat inthe cable causes the powder coating to melt and cure.

The following examples illustrate the present invention and are notintended to limit the invention in any manner.

EXAMPLE 1

A thermosetting powder coating composition in accordance with thisinvention was prepared from the components indicated in Table 1.

                  TABLE 1                                                         ______________________________________                                        COMPONENT          PARTS BY WEIGHT                                            ______________________________________                                        Epoxy resin (EPON Resin 1007F,                                                                   50.0                                                       epoxide equivalent weight                                                     about 2000)                                                                   Epoxy resin (EPON Resin 1009F                                                                    50.0                                                       epoxide equivalent weight                                                     about 2800)                                                                   Epoxy resin (EPON Resin 1004,                                                                    10.0                                                       epoxy equivalent weight                                                       about 910)                                                                    Masterbatch of 95 wt % 2-                                                                        0.6                                                        methylimidazole/5 wt %                                                        fumed silica                                                                  Acrylic resin (Resiflow P                                                                        1.5                                                        acrylic resin)                                                                Polyamide resin (Versamid 754                                                                    1.0                                                        polyamide resin)                                                              Colorant           0.8                                                        Fumed silica       0.1                                                        ______________________________________                                    

The above components, except for the fumed silica, were mixed togetherin a low intensity blender, melt mixed in an extruder, broken into chipsand then the chips were blended with the fumed silica in a low intensityblender. The resulting mixture was ground to powder in a hammermill andseived to a achieve the desired particle size.

EXAMPLE 2

A powder coating was prepared as in Example 1 using the components shownin Table 2.

                  TABLE 2                                                         ______________________________________                                        COMPONENT          PARTS BY WEIGHT                                            ______________________________________                                        Epoxy resin (EPON Resin 1007F,                                                                   50.0                                                       epoxide equivalent weight                                                     about 2000)                                                                   Epoxy resin (EPON Resin 1009F,                                                                   50.0                                                       epoxide equivalent weight                                                     about 2800)                                                                   Masterbatch of 95 wt % 2-                                                                        0.6                                                        methylimidazole/5 wt %                                                        fumed silica                                                                  Acrylic resin (Resiflow                                                                          1.5                                                        P acrylic resin)                                                              Polyamide resin (Versamid 754                                                                    1.0                                                        polyamide resin)                                                              Colorant           0.8                                                        ______________________________________                                    

Cable coatings are normally applied at film thicknesses in the range of25 to 35 mils or 15 to 20 mils. The powder coatings prepared in Examples1 and 2 were applied to cable at film thicknesses up to and includingthese ranges and demonstrated excellent flexibility and elongationcharacteristics, resulting in essentially crack-free coatings, evenafter tensioning the cable.

What I claim is:
 1. A process for protecting a metal cable againstcorrosion comprising applying to the cable a thermosetting powdercoating composition comprising about 80 to about 130 parts by weight ofan epoxy resin or mixture of epoxy resins having an epoxide equivalentweight of at least about 2250, about 0.1 to about 3.0 parts by weight ofa cure catalyst for the epoxy resin, about 0.5 to 3.0 parts by weight ofan acrylic resin, and about 0.5 to about 10.0 parts by weight of apolyamide resin, and curing said powder coating, whereby an essentiallycrack-free coating is formed on the cable.
 2. The process of claim 1wherein, following curing of the powder coating, the cable is tensionedand the coating remains essentially crack-free.